Since the discovery of high-temperature superconducting (HTS) materials (material that can retain its superconducting properties above the liquid nitrogen temperature of 77 K) there have been efforts to research and develop various engineering applications using such HTS materials. In thin film superconductor devices and wires, most progress has been made with fabrication of devices utilizing an oxide superconductor including yttrium, barium, copper and oxygen in the well-known basic composition of YBa2Cu3O7-y (hereinafter referred to as Y123 or YBCO) which remains the preferred material for many applications, including cables, motors, generators, synchronous condensers, transformers, current limiters, and magnet systems for military, high energy physics, materials processing, transportation and medical uses.
Even though Y123 is the material of choice for HTS applications, improvements in critical current density, in particular, critical current density in high magnetic fields and temperatures (Jc(H,T)), would reduce application costs, and reduce the size and weight of the system. Hence, it has been important to continue to improve the performance of Y123 superconductors.
One method to achieve such improvements includes “pinning” of the superconducting vortices, which is thought to be the underlying mechanism for high critical current density Jc in HTS materials. To achieve pinning in superconductors, matching the local potential energy differences as closely as possible to the size of the normal core of the superconducting flux line or vortex have been attempted. It is generally thought that the cross-sectional core has a size on the order of the coherence length (which is several nanometers in high temperature superconducting cuprates and grows with temperature). Moreover, it is thought that critical currents in polycrystalline high-temperature superconductors are still further reduced by weak links at the grain boundaries, which are made worse by high porosity, misalignment of the crystalline axis of adjacent grains, and by formation and accumulation of non-superconductor phases (compounds) at boundaries between superconducting grains.
There have been efforts to improve flux pinning of bulk superconducting materials. For example, the superconducting properties of YBa2Cu3O7-x compounds with partial substitutions with europium (Eu), gadolinium (Gd), and samarium (Sm) were found to show a slight improvement in intragrain Jc (flux pinning). The doping of YBa2Cu3O7-x with a wide range of dopants at the Y, Ba and Cu sites were also reported. Increased density of twin boundaries was also reported to provide only moderate improvement in flux pinning. Some increase in Jc in YBa2Cu3O7-x was achieved by the introduction of point defects by neutron irradiation.
In attempting to form thin films of Y123 compositions with enhanced flux pinning, yttrium-based impurities such as Y2O3 and Y2BaCuO5 (Y211) in Y123 thin films have been formed. Moreover, a multilayer thin film superconducting composition having alternating layers of two rare earth materials, (RE)123, was also reported. Lastly, RE-123 thin films containing a mixture of rare earth elements was also reported.